J. Am. Chem. Soc. 1996, 118, 8459-8469
8459
Poly(oxyethylene)-Water Interactions: A Molecular Dynamics Study Kenzabu Tasaki† Contribution from the Department of Chemistry, Washington UniVersity, Campus Box 1134, St. Louis, Missouri 63130 ReceiVed March 28, 1995. ReVised Manuscript ReceiVed May 6, 1996X
Abstract: Molecular dynamics simulations of a poly(oxyethylene) (POE) chain with 15 ethylene oxide (EO) units have been performed in an aqueous solution for 2 ns at 300 K and for 1 ns at 373 K, a cloud point for POE. The conformation and the hydration structure of POE and the structure and the dynamics of water molecules in the vicinity of POE were examined. The conformation of POE was transformed from a collapsed chain in the gas phase to a helix in water which was maintained for 2 ns. After a simulated annealing at 1000 K, POE still showed a preference for a helix. An extended network of POE-water and water-water hydrogen bonds was found throughout inside the helix, stabilizing the helix backbone. The same helical conformation was maintained even at 373 K. The pair distribution functions for water oxygen atoms near POE indicated significantly enhanced water structures in the hydrophilic region of POE, and to a lesser extent in the hydrophobic region, at both 300 and 373 K. Considerably reduced translational movements of water molecules near POE were also observed at both temperatures. All the results obtained from the simulation are in good agreement with experiments. We discuss the solubility and the phase behavior of POE in water, based on the observation from the simulations.
Introduction The nature of water-soluble polymers in aqueous solutions is closely related to the interactions with water. In fact, the characteristics of some water-soluble polymers are very sensitive to the solvent.1 Poly(oxyethylene) (POE),2 -[CH2CH2O]n-, is one of those polymers. Our NMR study of POE4 demonstrated a significant conformational change in going from organic solvents to water, which is supported by various spectroscopic measurements.5 Our primary interest is two-fold: the POE’s conformational behavior in water and the water structure in the vicinity of POE. We discuss these issues in relation to the solubility and the phase behavior of POE in water. POE has a unique solubility in that POE dissolves in water to any extent. This characteristic stands in sharp contrast to water solubility of other polyethers such as poly(oxypropylene), -[CH2CH(CH3)O]n-, and poly(oxytrimethylene), -[CH2CH2CH2O]n-, insoluble in water with different degrees of hydrophobicity. It is thus of significant interest to examine the interactions of both hydrophilic and hydrophobic groups of polyethers with water in order to understand the function of each group in the miscibility of polyethers in water. Also the subjects of our report are the role of water in solvation of a polyether and the relationship between the solubility and the conformation in water. It is still not clear to what extent † Present address: Mitsubishi Chemical America, 99 W Tasman Dr., Suite 200, San Jose, CA 95134. X Abstract published in AdVance ACS Abstracts, August 1, 1996. (1) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and BehaVior; CRC Press: Boca Raton, 1983; Vol. 1, p 19. (2) According to J. M. Harris, poly(ethylene glycol) refers to polyols with molecular weight
